This invention relates to the use of nonlinear optical photorefractive materials to combine information, and further relates to the control of the flow of information.
When two coherent beams of light intersect, their wave nature results in an interference pattern or gating pattern. By placing a photorefractive crystal at the location of intersection, this interference pattern can be made to form within the crystal. The interference pattern is made up of a stationary pattern of alternating bright and dark fringes of light. If the two beams have the same intensity and if they are mutually coherent, there will be no light in the dark fringes. Using light of a suitable wavelength for the given photorefractive material results in the excitation of charge carriers within the bright fringes. These charge carriers are mobile, and they diffuse in all directions. They can be captured by charge acceptors in the regions both bright and dark fringes, but when they are captured in the regions of dark areas, there is a very small probability of re-excitation. For this reason an alternating fringe pattern of plus and minus charge forms. The index of refraction of the crystal is related to its electronic properties, and this charge grating produces an index of refraction grating. The two beams of light effect a variation in the index of refraction of the material, hence the name "photorefractive." A third beam of light can be efficiently diffracted from the grating if it is incident on the grating at the Bragg angle. The Bragg angle is the angle at which the weak reflections from each small refractive index variation add together coherently, resulting in a large diffracted signal.
Some of the literature relating to photorefractive materials and their applications are, for example, Topics in Applied Physics Photorefractive Materials and Their Applications I Fundamental Phenomena, P. Giinter and J. P. Huignard, Eds., Springer-Veriag, Berlin, 1988. and Topics in Applied Physics Photorefractive Materials and Their Applications II Survey of Applications, P. Giinter and J. P. Huignard, Eds., Springer-Verlag, Berlin, 1988. and the references therein. Photorefractive materials have already been used to combine (and subtract) information from two beams. They have also been used to Bragg-diffract a third beam off of a grating formed by two other mutually coherent beams of light.
Thus, there exists a need for a device which combines several input beams with information therein to produce a selective output beam.